LT8228 datasheet(39/69 Pages) AD

39 / 69 page

LT8228

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 11. V1 Output Current Limit Programming at ISET1P

In buck mode, CSA1’s feedback current is generated out

of the ISET1P and IMON1 pins for input current regulation

and monitoring. In boost mode, CSA1’s feedback current

is generated out of the ISET1N, ISHARE and IMON1 pins

for output current regulation, sharing and monitoring.

Refer to the ISET1P, ISET1N, ISHARE and IMON1 gain

error curves in the Typical Performance Characteristics

section for more information.

High RSNS1 values improve current sense accuracy while

low RSNS1 values improve efficiency. Input referred off-

set voltage of CSA1 is guaranteed across temperature

to ±0.5mV at 50µA feedback current. Select the value of

RSNS1 so that the input referred offset voltage does not

affect current sense accuracy while minimizing power

loss. The current through RSNS1 is discontinuous in both

buck and boost mode. the voltage across RSNS1 is high-

est at the peak inductor current. However, power loss at

the sense resistor depends on the peak inductor current,

power stage duty ratio, and the capacitors at the RSNS1

terminals. ADI recommends a RSNS1 value that sets the

maximum voltage across RSNS1 to a value between 50mV

to 200mV. The power dissipation at the current sense

amplifier should not exceed its power rating for all oper-

ating conditions.

Next, select RIN1 to set the gain of the V1 current sense

amplifier to maintain the following condition for current

sense accuracy (see Equation 24).

RIN1 >

ISNS1(MAX) • RSNS1

72.5µA

(24)

Additionally, the current sense resistor RSNS1 and input

gain resistors RIN1 are used to sense BG or TG MOSFET

short fault. A short fault current is detected when ICSA1

reaches 120µA typically. Anytime such a fault is detected

through RSNS1, the LT8228 shuts down all four external

N-channel MOSFETs, pulls the SS pin low, asserts the

FAULT pin, IGND goes high impedance and reports the

status at the REPORT pin. The part restarts after waiting

1024 switching clock cycles. The CSA1 is internally com-

pensated. Any capacitive load at the SNS1P and SNS1N

affects the feedback compensation of the amplifier and

makes it unstable.

RSET1P SELECTION FOR V1 INPUT CURRENT LIMIT

(BUCK MODE)

In buck mode, V1 input current limit is programmed by

connecting a resistor RSET1P from ISET1P to ground.

The V1 current sense amplifier CSA1 outputs current at

the ISET1P pin that is proportional to the current ISNS1

flowing through the sense resistor RSNS1 as shown in

Figure 11. The current through the sense resistor is equal

to the V1 input current for V1 input currents much higher

than CSA1’s input bias and feedback currents as stated

in the RSNS1 and RIN1 Selection section.

During current limit, the LT8228 regulates the voltage at

the ISET1P pin to the typical internal reference voltage

of 1.21tV. For V1 output current limit IV1P(LIM), calculate

RSET1P according to Equation 25.

RSET1P =

RIN1 • 1.21V

RSNS1 •IV1P(LIM)

(25)

RIN1

RSET1P

CC2

RC2

CSET1P

RSNS1

SNS1P

SNS1N

CSA1

LT8228

IV1

TO M2

ISET1P

ISET2P

1.21V

ERROR AMPLIFIER 2

EA2

FB2

VC2

MAX

MIN

ISNS1

IB1

ICSA1 + IB1

ICSA1

VIREF

V1D

8228 F11

ISET1N =

ISNS1•RSNS1

RIN1

IV1=ISNS1–IB1

IV1N(LIM) =

RIN1•1.21V

RSNS1•RSET1N

FOR IV1>>IB1

VIREF IS LOWER OF THE VSS AND 1.21V

LT8228 Datasheet(PDF) 39 Page - Analog Devices

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